docs: def get_feature_variable_double(self, feature_key, variable_key, user_id, attributes=None):
      """ Returns value for a certain double variable attached to a feature flag.
  
      Args:
        feature_key: Key of the feature whose variable's value is being accessed.
        variable_key: Key of the variable whose value is to be accessed.
        user_id: ID for user.
        attributes: Dict representing user attributes.
  
      Returns:
        Double value of the variable. None if:
        - Feature key is invalid.
        - Variable key is invalid.
        - Mismatch with type of variable.
      """
  
      variable_type = entities.Variable.Type.DOUBLE
      return self._get_feature_variable_for_type(feature_key, variable_key, variable_type, user_id, attributes)